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Components and CircuitsFor discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

I once saw a cascade of blown fuses ranging from a 3A one in the plug to a 30A MCB protecting the subcircuit. I find it difficult to believe that the energy let through by the lowest rated fuse (usually quoted as I*I*t) was enough to trip or blow the bigger ones but that's what happened.

The original fault was a mains filter cap that exploded in a colleague's face. Fortunately he was unhurt. This all happened at BBC Kingswood Warren way back in 1974.

My experience is that MCBs are much faster than fuses and so spoil the blowing-chain.

My big variac can blow any MCB through the smallest fuse due to the very short but huge amplitude switch-on surge. I actually fitted a smaller MCB to it in the hope that it would beat the ring 30A but in practice they both trip together.

There are curves published for all types of fuses and MCB's. Basically the curve is a trace of current verses disconnection time and competent designers can look at the curves (normmally using both curves on one graph) and see if there are any overlaps.

If there is an overlap, it's possible for the higher rated fuse/MCB to blow before the lower rated one.

For simple house wiring and small industrial systems there are many computer progs. which do it all for you...you just type in the cable sizes, the known or measured impedances and the size/type of the chosen protection device and low and behold, the print out lets you know the current (ex. pun) situation.

For large systems and those getting close to supply transformers it's often a good idea to do a manual check.

A blown fuse in a plug top is one thing but a wrong setting on a 5000A ACB which could cause pieces of 100mm X 20mm copper bars to vaporise and blow a piece of sheet steel weighing 20Kg over 100 yds is another.

There are about 4 different types of MCB ranging from the quick blow to ones with operating curves designed to cope with huge in-rush currents. Standard house type are designed to trip quicker than fuses.

Fuses and MCBs only protect the wiring 'upstream' of them in order to avoid fires and damage (who wants to rewire everything?) - they are only intended to perform that function. Your equipment can go fry as far as the power distribution cares, that's all the way up your plug-top fuse!

Take a mains radio with a 3 Amp fuse for example, lets imagine that it develops a fault that causes it to draw 2.8A rather than the normal 0.4A. While your radio is busy burning furiously, the wiring up to and including the fuse is happily (and safely) operating normally within all its design limits. The fuse didn't protect your radio and your electric bill is growing larger.

The 'safety' gap between 'normal' operating current and fuse popping current is always difficult to assess absolutely as there are usually surges associated with switch on (e.g. charging capacitors) and even normal running (e.g. spinning up disc drives). The difference between the normal minimum and maximum operating currents is one reason for many fuse and MCB variants.

If you know your equipment will draw 20A for 2ms after switch on, then settle to 0.4A, the only fuse that is guarenteed not to nuisence trip is 20.1A. But the prospect of this fuse supporting a fault current of 18A may be a little unnerving! So you would fit a slow-blo 1A fuse (for example). Better to fit an MCB though since fuse wire heating and cooling (through the current spike) causes the fuse wire metal to expand and contract, thus causing metal fatigue and accelerating failure.

If you know your equipment will draw 20A for 2ms after switch on, then settle to 0.4A, the only fuse that is guarenteed not to nuisence trip is 20.1A.

This is an oversimplification. As indicated by Peter Munro in an earlier post the really important characteristic of a fuse under surge or short circuit conditions is its "let through" energy. Commonly written as "I squared t". This will be larger for a higher rated fuse and also for a slow blow fuse. The fuse should be chosen to not let through hazardous amounts of energy under these conditions yet not blow under normal surges. Long term heating effects are not important here as the fuse should blow pretty fast if there is this kind of fault.

The normal overload characteristics are rather different. It is assumed that thermal equilibrium is attained and the fuse should blow before anything else overheats dangerously. This can give very real problems, since fuses will take a long time to blow on small overloads (up to 50% or sometimes even more) during which time things can get a bit too hot.

I believe that the standard mains fuse values were carefully chosen so that the smallest one in a chain usually blows first.

It's about a factor of 3 to get this effect, hence 100A master with 30A ring and 13A on the plug and even 3A or 5A plugged into an extension.

I've seen a similar layout ,with different values though on telecomms transmission racks .The unusual thing we found was that where you'd expect the smallest to go first , at times it would be some way down the chain that one would go .This was with the manufacturers rated fuses in all places .Asked power staff to explain - they couldn't ,but had experienced it themselves .

would a 1A fuse with a 250V rating still work in a 12V position?
Probably not. It has never worked for me, in fact I have spent much extra time troubleshooting circuits just because I used a 125v or 250v fuse in an automotive circuit.

The voltage rating of a fuse is the highest voltage at which it can be used safely. It's also usually an AC rating. The DC voltage rating will be much lower because a DC arc does not self extinguish at the zero crossovers. However a 125V or 250V fuse will be perfectly safe at 12V DC. It will not be safe at 250V DC.

Automotive fuses are rated in a somehwat different way to other types. I'll leave the explanation to those who understand this better but it may be true that a 5A automotive fuse is roughly equivalent to a 10A normal fuse.

Finally there's the surge rating. I suspect that automotive fuses are rated for pretty high surge currents. This suggests you should use "antisurge" or "slo-blo" types.

I've seen a similar layout, with different values though on telecomms transmission racks .The unusual thing we found was that where you'd expect the smallest to go first , at times it would be some way down the chain that one would go .This was with the manufacturers rated fuses in all places .Asked power staff to explain - they couldn't ,but had experienced it themselves .

If you had for example a table lamp with a 3A fuse in its plug, plugged into an extension lead with a 7A fuse, plugged into another extension lead with a 13A fuse, plugged into a ring main with a 30A fuse. Then you (or some other idiot) cuts through the table lamp's cable close to the plug. The fault current could be several hundred amps, more than enough to blow any of the fuses, and this current will flow until one of the fuses blows.

You might reasonably expect the smallest rated fuse to blow first, its thinner wire will have a smaller thermal mass, but there's no guarantees, and the other fuses may have been weakened. So when you demonstrate what you did wrong, and cut through the rewired, re-fused lamp cable again with your new un-melted cutters, a different fuse may blow this time!

There are curves published for all types of fuses and MCB's. Basically the curve is a trace of current verses disconnection time and competent designers can look at the curves (normmally using both curves on one graph) and see if there are any overlaps.

Very true, See:

Quote:

To ensure discrimination is a very complicated matter, particularly where an installation includes a mixture of types of fuse, or of fuses and circuit breakers. Manufacturers' operating characteristics must be studied to ensure discrimination. As a rule of thumb where fuses or circuit breakers all of the same type are used, there should be a doubling of the rating as each step towards the supply is taken.

When fault current is high enough to result in operation of the protective device within 40 ms (two cycles of a 50 Hz supply), the simple consideration of characteristics may not always result in correct discrimination and device manufacturers should be consulted.

When RCDs are connected in series, discrimination between them is also important, the rule here being that a trebling in rating applies with each step towards the supply

Extract taken from one of my electrical documents, as I thought this would be useful, and to dispel some myths.
Emphasis mine to highlight the point

This is why for example, a 5 Amp plug/spur (BS1362) fuse may blow and the 3 Amp plug fuse downstream may not, or they may both blow, ie. a 3Amp plug fuse will not discriminate against a 5 amp plug

cheers,
Baz

__________________
I don't suffer from Insanity. I enjoy every minute of it.

I know that what I am going to say is a little bit OT.
My worry is some imports from the far east. For years I have come across CD players, Touch Lamps some small LCD TV sets and other items with NO internal mains fuse. Yes there can be fuses after the bridge rectifier or mains transformer, but so many items sold here are relying on the fuse in the plug, often the rating is far in excess of the current drawn.
Now I have heard some saying here the plug fuse is to protect the mains cable only, this is total rubbish. Irons, Kettles, Toasters, Washing machines, fires and drills just do not have internal fuses so they rely on the plug fuse for shorted elements, seized motors etc, not just knackered cables.

My worry though is the smaller stuff less internal low rated fuses, assume a unit with a mains tranny with a fault that allows it to draw 1.5a from the mains, a good manufacturer would fit a 800ma fuse internally but all there is, is a 3 amp in the plug, so the mains TX overheats and cooks, at the best it blows the thermal fuse on the tranny at worst it causes a fire. So we see that the fuse in the plug is worthless it protects nothing. So please don't say to me that the plug fuse is ONLY to protect the cable it isn't, it all depends what you have at the end of the cable!

Manufacturers need to seriously look at what they make and how they protect the appliance and the end user.

The thing to remember is that a fuse does not LIMIT the magnitude of the current.

It does not protect anything, including other fuses in the circuit, from surge current damage.

All it does is limit the duration of the current to prevent serious damage (eg fires!). Up to the rated current, the fuse should last indefinitely. At twice the rated current, the fuse should blow in less than - depending on the type of fuse (fast or slow-blow etc) - 60 seconds or so. For higher currents, the fuse will blow progressively faster.

In places where the fault current can be very high, eg on the mains side of equipment, you must have a HRC fuse in the circuit because normal glass fuses may not break very high fault currents. In the UK, this fuse is in the 13A mains plug, and will break the very high currents you would get with a short circuit to the live wire.

On UK mains, a 3A mains fuse will allow a dissipation of 750W continuously or 1.5kV for up to a minute, too high to protect most small appliances. With the HRC fuse in the plug, fuses on the mains input side of appliances do not need to be HRC types, and can be normal glass fuses with a low current rating chosen to protect the equipment.

I had the voltage rating on fuses explained to me by the local supplier thusly:
The voltage rating on the fuse is the highest voltage that the fuse will take after it opens and not arc over.
So its fine to use a higher voltage rated fuse on lower voltage but not the reverse.

That being said, one must keep in mind that the $35.00 semiconductor will blow to protect the $.25 fuse.

Here in the States, the ceramic sand filled fuses are generally used in high heat applications, according to my distributor. Typical use for them here is in microwave ovens.

The voltage rating for a fuse is the max amount of voltage it will take before arcing across the opened fuse metal ends.
IE: A 32V car fuse, can be used on the mains voltage, but if it fails, the voltage can arc over the ends of the broken fuse ends.
If a fuse of a 250V rating is used, it is supposed to not arc till the voltage exceeds 250 V.

I have some big industrial fuses here that one can unscrew and one can replace the fusing wire element inside them when they blow. It can take a pretty good load to open a 1000 A fuse.
(Hopefully its not a "body resistance test" type load. I saw way too many examples of that working for the state labor & industries safety division in my youth.)

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